(defpackage :interest
  (:use :cl))

(in-package :interest)

(defun simple-balance (principal periods interest-rate)
  (* principal (+ 1 (* periods
		       (rationalize interest-rate)))))

(simple-balance 100 3 1/5)

(defun compound-balance (principal periods interest-rate)
  (* principal (expt (+ 1 (rationalize interest-rate))
		     periods)))

(compound-balance 100 3 1/5)